1. Field of the Invention
This invention relates to a method of detecting radiation energy stored in an amount exceeding a predetermined amount on a stimulable phosphor sheet in the course of reading out a radiation image stored on the stimulable phosphor sheet by scanning the stimulable phosphor sheet with stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal by a photodetector, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
As disclosed in, for example, U.S. Pat. No. 4,400,619 and Japanese Unexamined Patent Publication No. 56(1981)-12599, the stimulable phosphor sheet can be used repeatedly by releasing therefrom the radiation energy remaining after the radiation image read-out by exposure to light or heat.
However, in the case where the level of the residual radiation energy is very high, the radiation energy often remains on the stimulable phosphor sheet even though the stimulable phosphor sheet is exposed to erasing light for releasing the residual radiation energy therefrom. Accordingly, based on the finding that the level of the residual radiation energy is proportional to the level of the radiation energy stored on the stimulable phosphor sheet in the image recording step, it has been proposed in, for example, Japanese Unexamined Patent Publication No. 58(1983)-80633 to control the erasing light irradiation amount on the basis of the amount of light emitted by the stimulable phosphor sheet in the image read-out step (the amount of the emitted light is proportional to the level of the radiation energy stored on the stimulable phosphor sheet).
On the other hand, as the apparatus for reading out the radiation image, an apparatus for scanning the stimulable phosphor sheet with stimulating rays and detecting the light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy by use of a photomultiplier has heretofore been used widely. However, in the case where the photomultiplier is used as the photodetector for detecting the light emitted by the stimulable phosphor sheet, the read-out gain of the photomultiplier is adjusted as suitable for the read-out of a normal radiation image. Therefore, in the case where the radiation energy was stored on a very high level on the stimulable phosphor sheet, it often occurs that the output current of the photomultiplier saturates. In this case, it is not always possible to accurately control the erasing light irradiation amount by ascertaining the level of the residual radiation energy from the level of the radiation energy which was stored on the stimulable phosphor sheet in the image recording step.
In view of the above circumstances, the applicant proposed in Japanese Unexamined Patent Publication No. 60(1985)-260035 a method capable of accurately detecting the level of the radiation energy stored on a stimulable phosphor sheet, and consequently the level of the residual radiation energy, and accurately controlling the erasing light irradiation amount even in such a case that the output current of the photomultiplier saturates. The proposed method comprises the steps of detecting the level of the radiation energy stored on the stimulable phosphor sheet from the voltage generated at a bleeder resistor of the photomultiplier, and controlling the erasing light irradiation amount in accordance with the voltage generated at the bleeder resistor. The proposed method is based on the finding that, even after the output current of the photomultiplier saturates or approaches the saturated condition, the voltage generated at the bleeder resistor decreases with sufficient sensitivity as the amount of light received by the photomultiplier increases.
With the proposed method, basically, the erasing light irradiation amount can be controlled accurately. However, in the case where the light guiding optical system for guiding the light emitted by the stimulable phosphor sheet to the photomultiplier or the photomultiplier itself exhibits shading (i.e. the problem that a portion where the light guiding efficiency or the light receiving efficiency is low is present locally), the amount of the light by the stimulable phosphor sheet and guided or received through said portion is detected as a light amount lower than the actual light amount. Therefore, the level of the radiation energy stored on the stimulable phosphor sheet is detected incorrectly.